Wires of metals such as tungsten, copper, iron, molybdenum, and stainless steel are produced by drawing the metals through diamond wire dies. Diamond wire dies have been fabricated using single crystal diamonds, however, such dies are difficult to fabricate, tend to chip easily, easily cleave, and often fail catastrophically in use because of the extreme pressures involved during wire drawing.
With reference to single crystal diamond wire dies, it is reported in Properties and Applications of Diamond, Wilks et al, Butterworth-Heinemann Ltd 1991, pages 505-507: "The best choice of [crystallographic] direction is not too obvious because as the wire passes through the die its circumference is abrading the diamond on a whole 360.degree. range of planes, and the rates of wear on these planes will be somewhat different. Hence, the originally circular hole will not only grow larger but will loose its shape. However, &lt;110&gt; directions offer the advantage that the wire is abrading the sides of the hole with {001} and {011} crystallographic orientations in abrasion resistant directions."
Diamond dies which avoid some of the problems attendant with natural diamonds of poorer quality comprise microporous masses compacted from tiny crystals of natural or synthesized diamonds or from crystals of cubic boron nitride. The deficiencies of such polycrystalline hard masses, as indicated in U.S. Pat. No. 4,016,736 to Carrison et al., are due to the presence of micro-voids/pores and soft inclusions. These voids and inclusions can be more than 10 microns in diameter. The improvement of Carrison et al. incorporates an impregnated lubricant in the microporous wire die and a metal cemented carbide jacket to enclose the die.
European Patent Application 0 494 799 A1 describes a polycrystalline CVD diamond layer having a hole formed therethrough and mounted in a support. As set forth in column 2, lines 26-28, "The relatively random distribution of crystal orientations in the CVD diamond ensures more even wear during use of the insert." As set forth in column 3, lines 50-54, "The orientation of the diamond in the polycrystalline CVD diamond layer 10 may be such that most of the crystallites have a (111) crystallographic axis in the plane, i.e. parallel to the surfaces 14, 16, of the layer 10."
Other crystal orientations for CVD films are known. U.S. Pat. No. 5,110,579 to Anthony et al describes a polycrystalline diamond film comprising substantially transparent columns of diamond crystals having a &lt;110&gt; direction perpendicular to the base, as illustrated in FIG. 3 of this patent.
As discussed in, for example, U.S. Pat. Nos. 5,361,621, 5,363,687 and 5,377,522, CVD diamond has been preferred for wire die applications because of its high purity and uniform consistency. Natural diamonds typically are less pure compositionally, and have less morphological consistency. Also, because CVD diamond usually can be produced without attendant voids, it is often more desirable than polycrystalline or single crystal diamond produced by high temperature and high pressure (HPHT) processes. However, the surfaces of CVD diamonds used for wire dies have been observed in some instances to contain to pits or voids after the polishing operations used to form the surfaces of the die, or after wire drawing. These pits or pores may result from the CVD deposition process or from pull-out of fine grains during these operations. Pull-out may result from relatively low grain boundary strength, which may in turn be related to the CVD deposition process. CVD deposited carbon films are known to contain hydrogen that is bonded to the carbon, particularly in the grain boundaries, which in turn results in a reduction in the number of carbon-carbon bonds across the grain boundaries, and hence, a reduction in the grain boundary strength. The degree of pitting observed is greater than that which occurs when natural diamonds that are subject to the same die forming operations.
Pits or pores are of concern, because they are expected to limit the maximum strength of wire dies in which they occur and, therefore, the types of wire that may be formed in them (e.g. relatively ductile alloys such as many copper alloys versus less ductile alloys, such as most tungsten alloys). Pits or voids may also cause defects in the drawn wire, particularly if they occur in the bearing surface of the die, where most of the wire deformation occurs.
Therefore, it is desirable to combine single crystal and CVD diamond to make composite diamond wire dies that take advantage of the properties and characteristics of both.